Well, shid old bloody toilets, they are to small, they leak, or bloody expensive systems.
There are decent sized bowls out there. Just can''t find my list of suppliers, is stowed some where in a file in the most far corner of my forepiek. However, what you are looking for is already stuff which is put in superyachts. Good size bowles with a efficent pump are often used in smaller commercial boats like tucks, pilot boats and so on. It might be an idea to have a look at the shipyards. Some of these systems can be converted in to a electric pump system. Befor you by one of these make sure they fit in your boat since the batrooms on yachts are usualy pretty small.
If you reconcider and want to look for good non leaking yacht size head, I recommand Jabsco. We are using one of their pump toilets on a 65 ft day charter cat with about 9000 guests using the toilet per saison and this more than once a trip. The complete toilet is in the third season and isn''t leaking a drop. Even this thing get cleand with bleach and desinfectant every day. Ok, the bowl is usual yacht size but at least it''s a good head. The only thing they don''t like is to be taken apart every second day to unblock them. That''s when they start to leak and you start using sikaflex to reseal them and it''s gettin worth every time you take them apart.